Measurement and correction of optical aberrations in charged particle beam microscopy

ABSTRACT

A charged particle beam microscope system is operated in a transmission imaging mode. During the operation, the charged particle beam microsystem directs a charged particle beam to the sample to produce images. A time series of beam tilts is applied in a pattern to the charged particle beam directed to the sample to produce a sequence of images. At least some of the images in the sequence of images are captured while the charged particle beam is transitioning between one beam tilt in the time series of beam tilts and a sequentially adjacent beam tilt in the time series of beam tilts. The pattern is configured to induce image changes between the images in the sequence of images that are indicative of optical aberrations in the charged particle beam microscope system.

FIELD

The field relates to charged particle beam microcopy.

BACKGROUND

Transmission electron microscopy (TEM) is a technique in which a beam ofelectrons is transmitted through a thin sample to form an image. Theimage is formed by the interaction of the electrons with atoms of thesample as the electrons are transmitted through the sample. The imagecan be captured by a camera. TEM is capable of producing high resolutionimages due to the small wavelength of the transmitted electrons.Sufficiently high resolution images can be used to investigate andanalyze the structural details of the sample on an atomic scale.However, optical aberrations in the microscope can limit the resolutionof the images.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system for measuring and correctingoptical aberrations in a charged particle beam microscope systemoperating in a transmission imaging mode, in accordance with variousimplementations.

FIGS. 2A and 2B are flow diagrams of a method for measuring andcorrecting optical aberrations in a charged particle beam microscopesystem operating in a transmission imaging mode, in accordance withvarious implementations.

FIG. 3A is a graph of a Lissajous figure beam tilt pattern.

FIG. 3B is a graph of an expected induced image shift pattern producedby the Lissajous figure beam tilt pattern depicted in FIG. 3A when thereis defocus in the charged particle beam microscope system.

FIG. 3C is a graph of an expected induced image shift pattern producedby the Lissajous figure beam tilt pattern depicted in FIG. 3A when thereis defocus and astigmatism in the charged particle beam microscopesystem.

FIG. 3D is a graph of an expected induced image shift pattern producedby the Lissajous figure beam tilt pattern depicted in FIG. 3A when thereis defocus, coma, and astigmatism in the charged particle beammicroscope system.

FIG. 3E is a graph of an expected induced image shift pattern producedby the Lissajous figure beam tilt pattern depicted in FIG. 3A when thereis defocus, coma, astigmatism, and spherical aberration in the chargedparticle beam microscope system.

FIG. 4A is a graph of a circular beam tilt pattern.

FIG. 4B is a graph of expected induced image shift produced by thecircular pattern depicted in FIG. 4A when there is defocus in thecharged particle beam microscope system.

FIG. 4C is a graph of expected induced image shift produced by thecircular pattern depicted in FIG. 4A when there is defocus andastigmatism in the charged particle beam microscope system.

FIG. 4D is a graph of expected induced image shift produced by thecircular pattern depicted in FIG. 4A when there is defocus, astigmatism,and coma in the charged particle beam microscope system.

FIG. 5A is a flow diagram of an example method of operating a chargedparticle beam microscope system with monitoring and correction ofaberrations.

FIG. 5B is a flow diagram of another example method of operating acharged particle beam microscope system with monitoring and correctionof aberrations.

FIG. 6 is a flow diagram of another example method of operating acharged particle beam microscope system with monitoring and correctionof aberrations.

FIG. 7 is a block diagram of a generalized computing system of asuitable computing environment that may be used in methods implementingmonitoring and correction of optical aberrations in a charged particlebeam microscope system.

DETAILED DESCRIPTION

The subject matter disclosed herein pertains to methods and systems thatmeasure and correct optical aberrations in charged particle beammicroscope systems, e.g., operating in a transmission imaging mode.Examples of optical aberrations that can be measured and correctedinclude, but are not limited to, defocus, two-fold astigmatism,three-fold astigmatism, four-fold astigmatism, five-fold astigmatism,sixfold astigmatism, axial coma, fifth order axial coma, sphericalaberration, sixth order spherical aberration, star aberration, sixthorder star aberration, three-lobe aberration, and rosette aberration.

In one or more implementations, a system includes a charged particlesource that emits a charged particle beam, an optical system defining anoptical axis and including one or more optical components configured toform the charged particle beam into a field of view of a sample disposedalong the optical axis, a pattern generator configured to generate apattern, one or more beam deflectors disposed along the optical axis andcontrollable by the pattern to apply a time series of beam tilts to thecharged particle beam, and an imaging sensor positioned along theoptical axis to capture images of the sample.

In one or more implementations, while operating the charged particlebeam microscope system in a transmission imaging mode and directing acharged particle beam to a sample, a time series of beam tilts isapplied in a pattern to the charged particle beam to produce a sequenceof images. At least some of the images in the sequence of images arecaptured while the charged particle beam is transitioning between onebeam tilt in the time series of beam tilts and a sequentially adjacentbeam tilt in the time series of beam tilts. In one or more examples, thesequence of images can be analyzed to determine optical aberrations inthe microscope system.

In one or more examples, the pattern is configured such that the beamtilts induce image changes (e.g., image shifts) in the sequence ofimages that can be correlated to optical aberrations in the microscopesystem. In one or more examples, image changes can be determined fromthe sequence of images and used to estimate one or more opticalaberration values for the microscope system. In other examples, one ormore optical aberration values can be determined by a trained neuralnetwork that receives the sequence of images or information derived fromthe sequence of images (e.g., power spectra of the images) as inputdata.

In one or more examples, the beam tilt pattern can be a periodicpattern, such as a Lissajous figure. The use of periodic patterns canmake it possible to determine an unknown time delay and unknowndeviation in frequency of the applied beam tilt pattern in relation tothe measured image changes over time. The periodic patterns can alsoenable a system that is robust against other effects that can induceimage changes (e.g., specimen drift and mechanical vibrations), allowingsignal processing to be used to separate the different signal sources.

The subject matter is described with implementations and examples. Insome cases, as will be recognized by one skilled in the art, thedisclosed implementations and examples may be practiced without one ormore of the disclosed specific details, or may be practiced with othermethods, structures, and materials not specifically disclosed herein.All the implementations and examples described herein and shown in thedrawings may be combined without any restrictions to form any number ofcombinations, unless the context clearly dictates otherwise, such as ifthe proposed combination involves elements that are incompatible ormutually exclusive. The sequential order of the acts in any processdescribed herein may be rearranged, unless the context clearly dictatesotherwise, such as if one act requires the result of another act asinput.

In the interest of conciseness, and for the sake of continuity in thedescription, same or similar reference characters may be used for sameor similar elements in different figures, and description of an elementin one figure will be deemed to carry over when the element appears inother figures with the same or similar reference character. In somecases, the term “corresponding to” may be used to describecorrespondence between elements of different figures. In an exampleusage, when an element in a first figure is described as correspondingto another element in a second figure, the element in the first figureis deemed to have the characteristics of the other element in the secondfigure, and vice versa, unless stated otherwise.

The word “comprise” and derivatives thereof, such as “comprises” and“comprising”, are to be construed in an open, inclusive sense, that is,as “including, but not limited to”. The singular forms “a”, “an”, “atleast one”, and “the” include plural referents, unless the contextdictates otherwise. The term “and/or”, when used between the last twoelements of a list of elements, means any one or more of the listedelements. The term “or” is generally employed in its broadest sense,that is, as meaning “and/or”, unless the context clearly dictatesotherwise. When used to describe a range of dimensions, the phrase“between X and Y” represents a range that includes X and Y. As usedherein, an “apparatus” may refer to any individual device, collection ofdevices, part of a device, or collections of parts of devices.

FIG. 1 illustrates a system 100, according to one implementation, thatcan measure and correct optical aberrations in a charged particle beammicroscope system while the charged particle beam microscope system isoperating in a transmission imaging mode. The system 100 is illustratedin the context of an exemplary charged particle beam microscope system104. However, the system 100 is not limited to the particularconfiguration of the charged particle beam microscope system 104 shownin FIG. 1 and could be used with any charged particle beam microscopesystem operating in a transmission imaging mode.

In the illustrated example, the charged particle beam microscope system104 includes a charged particle beam microscope column 120 (hereafter,microscope column 120) enclosed in a vacuum chamber 106. The microscopecolumn 120 defines an optical axis 116 of the charged particle beammicroscope system 104.

A sample 124 to be investigated can be disposed along the optical axis116. The sample 124 can be supported by a sample holder 140, which insome examples can have capabilities to translate, rotate, and/or tiltthe sample 124 relative to the optical axis 116. The sample holder 140can allow different areas of the sample 124 to be positioned relative tothe optical axis 116 for investigation.

At the top of the microscope column 120 is a charged particle source 108(e.g., a thermal electron source, Schottky-emission source, fieldemission source, etc.) that emits a charged particle beam 112 (e.g., anelectron beam). The charged particle beam 112 is directed along theoptical axis 116 such that it can impinge on the sample 124 and interactwith the sample 124. The charged particle beam 112 passes through thesample 124 to produce an image.

A camera 136 at the bottom of the microscope column 120 can be operatedto capture an image of the sample 124. The camera 136 can include, forexample, a CCD (charged-coupled device) imaging sensor, a CMOS(complementary metal-oxide-semiconductor) imaging sensor, or, moregenerally, an array of photodetectors. In one example, the camera 136can be operable in a “movie” mode to capture a sequence of images of thesample.

The microscope column 120 can include various charged particle beam lenssystems (hereafter, lens systems) that direct the charged particle beam112 along the optical axis 116 and configure the charged particle beam112 to provide a constant illumination covering a desired field of viewon the sample 124. In some examples, the various lens systems can alsoproject the image produced by interaction of the charged particle beam112 with the sample 124 into a field of view of the camera 136.

In some examples, the various lens systems can include a condenser lenssystem 122, an objective lens system 126, and a projector lens system130, which can have any suitable configuration to modify the chargedparticle beam 112 to provide a desired field of view. The condenser lenssystem 122 can be configured to condense the charged particle beam 112into a nearly parallel beam. The objective lens system 126 can beconfigured to focus the image formed by transmitting the chargedparticle beam 112 through the sample 124. The projector lens system 130can be configured to project the image coming from the objective lenssystem 126 to the field of view of the camera 136.

In some examples, the condenser lens system 122 can include condenserlenses 122 a, 122 b and other components, such as a condenser stigmator122 c (i.e., a device to correct axial astigmatism), and a condenseraperture 122 d.

In some examples, the objective lens system 126 can include an objectivelens 126 a and other components, such as beam deflectors 126 b (whichcan be used to center the beam), an objective stigmator 126 c (i.e., adevice to correct axial astigmatism), and objective aperture 126 d. Inthe illustrated example, the sample 124 is positioned above theobjective lens 126 a (in other examples, the sample 124 can be insertedinto the objective lens 126 a).

In some examples, the projector lens system 130 can include, forexample, an intermediate lens 130 a (e.g., to magnify the image) andprojector lenses 130 b, 130 c. The projector lens system 130 can includeother components, such as a stigmator, image deflector, and aperture.

The components of the lens systems can be electro-optical componentsthat are tunable by varying the electrical current applied to thecomponents. For example, the various lenses of the lens systems 122,126, 130 can be electromagnetic lenses. An electromagnetic lens modifiescharged particle beams by a magnetic field generated by a solenoid. Bychanging the electric current to the solenoid, the magnetic field can bechanged, which would correspondingly change the focal length andmagnification of the lens. The other components of the lens systems 122,126, 130, such as the stigmators and beam deflectors, can beelectromagnetic devices, which can be tuned in the same manner asdescribed for the electromagnetic lenses. The ability to tune thecomponents of the lens systems 122, 126, 130 by varying electricalcurrent supplied to the components can allow aberration corrections tobe applied to the microscope column 120 on the fly (e.g., while thecharged particle beam microscope system 104 is operating in atransmission imaging mode).

The lens systems 122, 126, 130 can include other optical components notshown in FIG. 1 or a different configuration of optical components. Themicroscope column 120 can include other optical components near theoutput of the charged particle source 108. For example, the microscopecolumn 120 can include an accelerator stack that can be used to increasethe acceleration of the charged particle beam 112 (e.g., prior tocondensing the charged particle beam 112).

The system 100 includes a pattern generator 144 that can generatevarious patterns. The pattern generator 144 can be, for example, aprogrammable function generator. In some examples, the pattern generator144 can be configured to generate periodic patterns. In one example, theperiodic pattern can be a Lissajous figure. Lissajous figure is given bythe following parametric equations:

x=Asin(at+δ)   (1)

y=Bsin(bt)   (2)

The Lissajous figure can be as simple as a circle (e.g., the parametricequations (1) and (2) give a circle when a/b=1, A=B, and δ=π/2 radians)or can be more complex (e.g., the ratio a/b is not equal to 1).

The system 100 includes beam deflectors 134 inserted in the microscopecolumn 120. The beam deflectors 134 can be controlled according to thepattern to tilt the charged particle beam 112 in the X and Y directionsover time. The beam deflectors 134 can apply tilt motion to the chargedparticle beam 112 prior to the charged particle beam 112 reaching thesample 124 (e.g., prior to condensing the charged particle beam 112).The beam deflectors 134 can be electro-optic deflectors (e.g.,electrostatic or magnetic beam deflectors), which would allow the tiltangles applied to the charged particle beam 112 to be changed by varyingthe electrical current to the beam deflectors.

The beam deflectors 134, operated according to a pattern, apply a timeseries of beam tilts to the charged particle beam 112. Each beam tiltproduces an image. Thus, the time series of beam tilts results in asequence of images. Image shifts between successive (or adjacent) imagesin the sequence of images can indicate optical aberrations in themicroscope system.

The system 100 can include a frame grabber 148 in communication with thecamera 136. The frame grabber 148 can, for example, form an interfacebetween the camera 136 and a computing system 154 (e.g., a server). Theframe grabber 148 is an electronic device that can capture individualimage frames from a stream of image data produced by the camera 136.

The system 100 can include an image shift tracker 152 in communicationwith the frame grabber 148. The image shift tracker 152 can receiveimage frames from the frame grabber 148 and measure (or compute) theimage shift between successive image frames over time. The image shifttracker 152 can use any suitable imaging processing technique to measureimages shift. One example technique is cross-correlation. In oneexample, the image shift tracker 152 can be executed by a processor(e.g., GPU, CPU, or FPGA) of the computing device 154.

The system 100 can include an aberration calculator 156 that determinesvalues for the optical aberrations in a charged particle beam microscopesystem based on the image shifts measured by the image shift tracker152. Examples of optical aberrations that can be found in chargedparticle beam microscopes and measured are defocus, two-foldastigmatism, three-fold astigmatism, four-fold astigmatism, five-foldastigmatism, sixfold astigmatism, axial coma, fifth order axial coma,spherical aberration, sixth order spherical aberration, star aberration,sixth order star aberration, three-lobe aberration, and rosetteaberration. The aberration calculator 156 can be configured to determinevalues for any combination of these optical aberrations as well as otheroptical aberrations not specifically mentioned.

In one example, the aberration calculator 156 can receive a stream ofimage shifts from the image shift tracker 152 and compute the opticalaberrations in the microscope column 120 based on the image shifts. Inone example, the aberration calculator 156 can estimate the opticalaberrations by fitting the image shifts to an optical aberration modelthat provides a relationship between image shifts, beam tilts, andoptical aberrations in the microscope column 120. In some examples, theoptical aberration model can be extended with one or more auxiliaryparameters that are also fitted. The auxiliary parameters can capturevariances in the measurements. For example, the time duration betweengenerating a tilt position and actually tilting the beam with the tiltposition can vary such that there is an unknown time delay between thebeam tilt applied and the sequence of images captured from applying thebeam tilt. In some examples, beam tilting control timing can beuncoupled or not synchronized with imaging sensors. In such cases, onesuch auxiliary parameter can be an unknown time delay or timesynchronization between the pattern that controls the beam tilt and theimages captured by the camera 136. Another example of auxiliaryparameters can include sample drift. With periodic tilt patterns, aphase of the periodic tilt pattern can be related to a phase of thedetected pattern to provide timing information. In some examples, aportion of a tilt pattern period, between one and two periods, or morethan two periods may be sufficient to provide time used to synchronizeclock timing between beam controls and image detection. Various tiltpatterns can be used, including non-periodic movements.

In another example, the aberration calculator 156 can determine valuesfor the optical aberrations based on optimization of a model thatprovides a relationship between a sequence of images and opticalaberrations in the microscope column 120. For example, an image acquiredat one tilt angle can be transformed to an image acquired at anothertilt angle. The transformation could be parameterized by variablesrelated to optical aberrations. These parameters would then correspondto the model. In an optimization routine, the optical parameters thatcan maximize the similarity of the transformed images can be found andused to determine values for the optical aberrations. This procedure canbe extended to multiple images.

In another example, the aberration calculator 156 can determine valuesfor the optical aberrations from an inference of a neural network thathas been trained to predict optical aberrations from image informationobtained by applying a series of beam tilts to a charged particle beamin a pattern. The trained neural network can receive the sequence ofimages as input and/or can receive information derived from the sequenceof images (e.g., power spectra of the images, image shifts, etc.) asinput and generate an inference, which can include a prediction andother information such as confidence of the prediction. In this example,the aberration calculator 156 can receive the image data directly fromthe camera 136 or receive image frames from the frame grabber 148,preprocess the image data, and provide the preprocessed image data tothe neural network. In some cases, the inference can be a service thatis provided via a cloud.

The system 100 can include an aberration corrector 168 that iscommunicatively coupled to the aberration calculator 156. The aberrationcorrector 168 can determine corrections to apply to the components ofthe microscope column 120 (e.g., the components of the lens systems)based on the optical aberrations determined by the aberration calculator156. For example, where the components are electro-optical components,the aberration corrector 168 can change the electrical current appliedto the components in order to change how the components modify thecharged particle beam 112. In some examples, the aberration corrector168 can apply the corrections to the components of the microscope column120 based on predetermined criteria (e.g., if the optical aberrationsexceed predetermined thresholds or if the confidence of an estimatedaberration value exceeds a predetermined threshold (e.g., if theabsolute value of an estimated aberration value is less than aconfidence interval for that optical aberration)).

In some examples, the aberration corrector 168 can determine adjustmentsto make to the pattern generated by the pattern generator 144 for beamtilting. For example, the aberration calculator 156 can provide theoptical aberrations with a level of confidence. If the level ofconfidence is low, the aberration corrector 168 can determine an optimalpattern to use in subsequent beam tilting in order to improve theconfidence of the measurements. The aberration corrector 168 can furtherdetermine the parameters of the optimal pattern (such as base frequencyand amplitude) and provide this information to the pattern generator144.

In some examples, the aberration calculator 156 and aberration corrector168 can be programs or scripts running on a computing system 160 that iscommunicatively coupled to the charged particle beam microscope system104. The aberration corrector 168 can be in communication with theaberration calculator 156 and the pattern generator 144. In someexamples, the computing system 160 can be coupled to a display 164(e.g., a console of the charged particle beam microscope system 104). Insome examples, the computing system 160 can present current aberrationvalues and trends in the aberrations on the display 164.

The image shift tracker 152, aberration calculator 156, and aberrationcorrector 168 can be implemented in software and/or hardware. Softwarecomponents of the image shift tracker 152, aberration calculator 156,and aberration corrector 168 can be stored in one or morecomputer-readable storage media or computer-readable storage devices andexecuted by one or more processors (e.g., processors in the computingsystem 160 or 154). Any of the computer-readable media herein can benon-transitory (e.g., volatile memory such as DRAM or SRAM, nonvolatilememory such as magnetic storage, optical storage or the like) and/ortangible.

FIG. 2A is a flow diagram illustrating a method 200 of measuring andcorrecting optical aberrations in a charged particle beam microscopesystem (e.g., the charged particle beam microscope system 104illustrated in FIG. 1 ) operating in a transmission imaging mode.Operations are illustrated once each and in a particular order in FIG.2A (and in the others of the flow diagrams discussed therein), but theoperations may be reordered and/or repeated as desired and appropriate(e.g., different operations performed may be performed in parallel, assuitable).

At 210, the method includes operating a charged particle beam microscopesystem in a transmission imaging mode. The operation 210 can includegenerating a charged particle beam (e.g., an electron beam) using acharged particle source at the top of the microscope column anddirecting the charged particle beam to a sample using optical componentsin the microscope column. The charged particle beam interacts with thesample to produce an image. The operation 210 can include projecting theimage into a field of view of a camera at the bottom of the microscopecolumn.

At 220, with the charged particle beam microscope system operating inthe transmission imaging mode as described in operation 210, the methodcan include applying a time series of beam tilts in a pattern to thecharged particle beam coming out of the charged particle source. Theoperation 220 can include generating the pattern. The operation 220 caninclude applying the time series of beam tilts by traversing thepattern. At each point on the traversal path, the x, y coordinates ofthe point determine the amount of tilt to apply to the charged particlebeam in the X and Y directions. The tilt amounts can be used to controlthe beam deflectors inserted in microscope column to tilt the beam. Insome examples, the pattern can be a periodic pattern. In some examples,the periodic pattern can be a Lissajous figure (or Lissajous curve).

At 225, while applying beam tilts to the charged particle beam asdescribed in operation 220, the method can include operating a camera ina movie mode to capture images of the sample. For every beam tilt, thecamera can obtain a full image of the sample. Thus, the camera cancapture a sequence of images corresponding to applying the time seriesof beam tilts to the charged particle beam. The time between applyingthe beam tilt and the actual tilting of the beam can vary such thatthere is an unknown time delay between the beam tilting pattern and thesequence of images captured. In some examples, the optical aberrationmodel can incorporate the unknown time delay as an auxiliary variable tobe fitted. Other auxiliary variables can be incorporated in the opticalaberration model as well, e.g., beam tilt pattern or sample drift.

At 230, the method can include estimating values for the opticalaberrations in the microscope column from the images captured inoperation 225. In one example, the optical aberrations can be estimatedfrom an optical aberration model that relates image shifts to beam tiltsand optical aberrations of the microscope column. In another example,the optical aberrations can be obtained by optimizing a model thatprovides a relationship between the sequence of images and opticalaberrations in the microscope column. In another example, the opticalaberrations can be obtained as an inference of a trained neural network.The inference can include the prediction of the neural network as wellas other information such as confidence of the prediction.

FIG. 2B illustrates one example of operation 230 of FIG. 2A. In theexample illustrated in FIG. 2B, at 240, the method includes determiningimage shifts from a sequence of images captured in operation 225. Theoperation 240 can include receiving image data from the camera andextracting a sequence of images (or image frames) from the image data.Each image will correspond to an image of the sample at a given beamtilt. Any shift between successive (or adjacent) images will depend onthe optical aberration of the charged particle beam microscope. Theoperation 240 includes determining these image shifts using any suitableimage processing technique, such as cross-correlation.

For illustrative purposes, FIG. 3A shows a beam tilt pattern 300 thatcan be applied to a charged particle beam, and FIGS. 3B-3E show expectedimage shift patterns 302, 304, 306, 308 produced from applying the beamtilt pattern 300 due to various types of microscope aberrations (e.g.,defocus, astigmatism, coma, and spherical aberration). The beam tiltpattern 300 is a Lissajous pattern (with a=3, b=2, where a and b areindicated in Equations (1) and (2)). FIGS. 3B-3E show the movement ofthe image of the sample over time as the beam tilt pattern is applied.If there are no optical aberrations in the microscope, the beam tiltpattern will not induce a shift in the images of the sample collectedover time. However, if there are aberrations in the microscope, thepattern formed by the movement of the sample will be different from theapplied beam tilt pattern (e.g., scaled and/or distorted).

The image shift pattern 302 shown in FIG. 3B corresponds to an examplewhere the optical aberration in the microscope is defocus. The imageshift pattern 302 and the applied beam tilt pattern 300 are linearlydependent (i.e., the image shift pattern 302 is scaled relative to theapplied beam tilt pattern 300−the amount of defocus is the scalingfactor between the figures shown in FIGS. 3A and 3B).

The image shift pattern 304 shown in FIG. 3C corresponds to an examplewhere the optical aberrations in the microscope are defocus andastigmatism. The image shift pattern 304 compared to the applied beamtilt pattern 300 is deformed in one direction (by astigmatism) andscaled (by defocus).

The image shift pattern 306 in FIG. 3D corresponds to an example wherethe optical aberrations in the microscope are defocus, astigmatism, andcoma. The image shift pattern 306 compared to the applied beam tiltpattern 300 is distorted asymmetrically (by coma), deformed in onedirection (by astigmatism), and scaled (by defocus).

The image shift pattern 308 shown in FIG. 3E corresponds to an examplewhere the optical aberrations in the microscope are defocus,astigmatism, coma, and spherical aberration. The image shift pattern 308compared to the applied beam tilt pattern 300 is distorted radially (byspherical aberration), distorted asymmetrically (by coma), deformed inone direction (by astigmatism), and scaled (by defocus).

FIG. 4A illustrates another example of a beam tilt pattern 400 that canbe applied to a charged particle beam, and FIGS. 4B-4D illustrateexpected image shift patterns 402, 404, 406 produced from applying thebeam tilt pattern 400 due to various types of microscope aberrations(e.g., defocus, astigmatism, and coma). The beam tilt pattern 400 is acircular pattern (which is a special case of a Lissajous figure). FIGS.4B-4D show the movement of the image of the sample over time as the beamtilt pattern is applied. If there are no optical aberrations in themicroscope, the beam tilt pattern will not induce a shift in the imagesof the sample collected over time. However, if there are aberrations inthe microscope, the pattern formed by the movement of the sample will bedifferent from the applied beam tilt pattern (e.g., scaled and/ordistorted).

The image shift pattern 402 in FIG. 4B corresponds to an example wherethe optical aberration in the microscope is defocus (the image shiftpattern 402 is scaled compared to the applied beam tilt pattern 400−theamount of defocus is the scaling factor between the figures shown inFIGS. 4A and 4B). The image shift pattern 404 in FIG. 4C corresponds toan example where the optical aberrations in the microscope are defocusand astigmatism (the image shift pattern 404 is scaled and deformed inone direction). The image shift pattern 406 in FIG. 4D corresponds to anexample where the optical aberrations in the microscope are defocus,astigmatism, and coma (the image shift pattern 406 is scaled, deformedin one direction, and distorted asymmetrically).

A circular pattern, such as the beam tilt pattern 400 shown in FIG. 4A,can capture low order aberrations (e.g., defocus) effectively but may beless sensitive to higher order aberrations. For higher orderaberrations, a more complex Lissajous figure, such as the beam tiltpattern 300 shown in FIG. 3A, may be more suitable.

Returning to FIG. 2B, at 245, the method includes estimating values forselected optical aberrations from the image shifts obtained in operation240. The aberration values can be estimated by fitting the image shiftsto an optical aberration model for the charged particle beam microscopesystem.

Equation (3) below is an example of an optical aberration model. Forillustrative purposes, the model of Equation (3) can obtain values fordefocus, astigmatism (astigX, astigY), coma (comaX, comaY), threefoldastigmatism (threefoldX, threefoldY), spherical aberration (spherical),star aberration (starX, starY), and fourfold astigmatism (fourfoldX,fourfoldY).

$\begin{matrix}{\begin{pmatrix}{shiftX} \\{shiftY}\end{pmatrix} = {\begin{Bmatrix}{\begin{matrix}{tx} \\{ty}\end{matrix}❘\begin{matrix}{tx} \\{- {ty}}\end{matrix}❘\begin{matrix}{ty} \\{tx}\end{matrix}❘\begin{matrix}{{tx}^{2} + \frac{{ty}^{2}}{y}} \\\frac{2{txty}}{3}\end{matrix}❘\begin{matrix}\frac{2{txty}}{3} \\{\frac{{tx}^{3}}{3} + {ty}^{2}}\end{matrix}❘\begin{matrix}{{tx}^{2} - {ty}^{2}} \\{{- 2}{txty}}\end{matrix}❘\begin{matrix}{2{txty}} \\{{tx}^{2} - {ty}^{2}}\end{matrix}❘--} \\{\begin{matrix}{{tx}\left( {{tx}^{2} + {ty}^{2}} \right)} \\{{ty}\left( {{tx}^{2} + {tx}^{2}} \right)}\end{matrix}❘\begin{matrix}{tx}^{3} \\{ty}^{3}\end{matrix}❘\begin{matrix}{\frac{1}{2}\left( {{3{tx}^{2}{ty}} + {ty}^{2}} \right)} \\{\frac{1}{2}\left( {{tx}^{2} + {3{txty}^{2}}} \right)}\end{matrix}❘\begin{matrix}{{tx}^{3} - {3{txty}^{2}}} \\{{{- 3}{tx}^{2}{ty}} + {ty}^{3}}\end{matrix}❘\begin{matrix}{{3{tx}^{2}{ty}} - {ty}^{3}} \\{{tx}^{3} - {3{txty}^{2}}}\end{matrix}}\end{Bmatrix} \cdot \begin{pmatrix}{defocus} \\{astigX} \\{astigY} \\{comaX} \\{comaY} \\{threefoldX} \\{threefoldY} \\{spherical} \\{starX} \\{starY} \\{fourfoldX} \\{fourfoldY}\end{pmatrix}}} & (3)\end{matrix}$

The vector on the right hand side of the equation contains values foroptical aberrations for a given state of the optical system of themicroscope system at a certain moment in time. When the tilt of the beamis adjusted with (tx, ty) radians, then the induced image shift will begiven by (shiftX, shiftY), which can be measured from images capturedwhile applying the beam tilts (e.g., in operation 240). The measuredshiftX, shiftY and known tx, ty can be fitted into the model of Equation(3) to obtain the estimated values for the optical aberrations. Theparameters tx, ty are time-dependent, which allows unknown time delaybetween the applied beam tilt pattern and the induced image shiftpattern to be factored into the model for fitting. Variations of theoptical aberration model can incorporate other auxiliary parameters,such as beam tilt pattern and/or sample drift.

Returning to FIG. 2A, at 250, the method includes determiningcorrections to apply to the optical components in the microscope columnbased on the aberration values estimated in operation 230 (or inoperation 245 in FIG. 2B).

At 260, the method can include applying the corrections determined inoperation 250 to one or more of the optical components in the microscopecolumn based on predetermined criteria. For example, if the estimatedvalues for one or more of the selected aberrations are abovepredetermined thresholds, the corrections could be applied.Alternatively, if the estimated values for the selected aberrations arebelow predetermined thresholds, the corrections may not be applied.However, the corrections can be stored and used during post processingof the captured images of the sample.

At 270, the method can include adjusting the pattern used in beamtilting in operation 220 based on predetermined factors. For example,the pattern can be adjusted, for example, if an accuracy of theestimated values for one or more selected optical aberrations is below apredetermined threshold, a quality of the images in the sequence ofimages is below a predetermined threshold, and/or there are externaldisturbances (e.g., mechanical vibrations) while capturing the sequenceof images. The pattern can be adjusted by changing the parameters of thepattern (e.g., the base frequency and/or amplitude of the pattern can bechanged) or by selecting a new pattern that is optimal for theconfiguration of the microscope and sample.

The operations 220-270 can be repeated until the end of a plannedacquisition or until the aberrations are measured with a desiredaccuracy or until aberrations are tuned to the desired values withdesired accuracy. The aberration measurements over time can be displayedon a screen and monitored.

FIG. 5A illustrates an example method 500 of operating a chargedparticle beam microscope system with monitoring and correction ofaberrations (e.g., the system 100 illustrated in FIG. 1 ). The method500 is optimized to accurately and precisely measure rapidly varyingaberrations. As noted above for FIG. 2A and the others of theaccompanying flow diagrams, operations are illustrated once each and ina particular order in FIG. 5A, but the operations may be reorderedand/or repeated as desired and appropriate (e.g., different operationsperformed may be performed in parallel, as suitable). For example, themethod 500 may include performing any suitable combinations of theoperations presented therein in parallel.

At 510, with a sample disposed along the optical axis of the chargedparticle beam microscope system, the method 500 includes moving to anarea of interest on the sample.

At 520, the method 500 includes measuring values of aberrations in thecharged particle beam microscope system. In one example, themeasurements can be made according to operations 210-230 in FIG. 2A.

At 530, the method 500 includes determining corrections for rapidlyvarying aberrations (e.g., defocus) in the charged particle beammicroscope system.

At 540, the method 500 includes monitoring changes in values for otheroptical aberrations (e.g., optical aberrations that are not rapidlyvarying, such as two-fold astigmatism, three-fold astigmatism, four-foldastigmatism, five-fold astigmatism, sixfold astigmatism, axial coma,fifth order axial coma, spherical aberration, sixth order sphericalaberration, star aberration, sixth order star aberration, three-lobeaberration, or rosette aberration). The changes in the opticalaberrations can be presented in a display, for example, in the form oftrend graphs.

At 550, the method 500 can include determining corrections for otheraberrations having values exceeding predetermined thresholds.

At 560, the method 500 includes applying any corrections determined inoperations 530 and 540 to the optical column of the charged particlebeam microscope system (or microscope column).

At 570, the method 500 includes acquiring an image of the sample withthe corrected charged particle beam microscope system. After acquiringthe image, the method 500 can return to operation 510 to process anotherarea of the sample.

Some optical aberrations (e.g., defocus) can be identified using arelatively small amount of data while other optical aberrations (e.g.,coma and astigmatism) can require a relatively large amount of data tobe appropriately identified. Instead of waiting to identify all theoptical aberrations before determining and applying optical correctionsto the charged particle beam microscope system, the optical correctionscan be determined and applied more quickly for the optical aberrationsthat can be identified using a relatively small amount of data.

FIG. 5B illustrates an example method 580 of operating a chargedparticle beam microscope system with monitoring and correction ofaberrations (e.g., the system 100 illustrated in FIG. 1 ) based on theprinciple of applying optical corrections at different frequencies. Asnoted above for FIG. 2A and the others of the accompanying flowdiagrams, operations are illustrated once each and in a particular orderin FIG. 5B, but the operations may be reordered and/or repeated asdesired and appropriate (e.g., different operations performed may beperformed in parallel, as suitable). For example, the method 580 mayinclude performing any suitable combinations of the operations presentedtherein in parallel.

At 581, a first set of one or more optical aberrations and a second setof one or more optical aberrations are determined. The first set of oneor more optical aberrations is different from the second set of one ormore optical aberrations (e.g., the one or more optical aberrations inthe first set are different than the one or more optical aberrations inthe second set). In one example, the differences between the first andsecond sets can be based on the amount of data required to appropriatelyidentify the optical aberrations and/or on the drift rate (i.e., thespeed at which the optical aberration significantly changes its value)in the microscope system. In one example, the first set can include oneor more optical aberrations that can be identified using a relativelysmall amount of data (e.g., defocus). In one example, the second set caninclude one or more optical aberrations that require a relatively largeamount of data to be appropriately identified (e.g., coma andastigmatism).

At 582, the method 580 includes moving to an area of interest on asample disposed along an optical axis of the charged particle beammicroscope system.

At 583, the method 580 includes acquiring images of the sample whileoperating the charged particle beam microscope system. The images can beacquired, for example, as described in operations 210-225 in FIG. 2A.

At 584, the method 580 includes determining a first optical aberrationoutput set for the charged particle beam microscope system. The firstoptical aberration output set includes values for the first set of oneor more optical aberrations in the charged particle beam microscopesystem. A first input set for determining the first optical aberrationoutput set includes one or more of the images acquired in operation 583.The values for the first set of one or more optical aberrations can bedetermined, for example, using the aberration calculator (156 in FIG. 1) and the first input set. In some examples, the first opticalaberration output set can further include values for one or more of thesecond set of one or more optical aberrations in the charged particlebeam microscope system. The values for the second set can be determined,for example, using the aberration calculator and the first input set.The values for the first set and second set can be determined within thesame aberration calculation (e.g., using the same optical aberrationmodel, optimization of a model, or trained neural network). The valuesin the first optical aberration output set can include estimated valuesof the optical aberrations and confidence of the estimation.

At 586, the method 580 includes applying corrections (or adjustments) tothe charged particle beam microscope system at a first frequency tomitigate the first set of one or more optical aberrations. Thecorrections to apply to the first set of one or more optical aberrationsare determined based on the values corresponding to the first set of oneor more optical aberrations in the first optical aberration output setdetermined in operation 584. In one example, the corrections can beapplied by varying electrical currents supplied to one or moreelectro-optical components of the charged particle beam microscopesystem.

At 588, the method 580 includes determining a second optical aberrationoutput set for the charged particle beam microscope system. The secondoptical aberration output set includes values for the second set of oneor more optical aberrations in the charged particle microscope system. Asecond input set for determining the second optical aberration outputset can include one or more of the images acquired in operation 583 orcan include some values from the first optical aberration output set(e.g., the values determined for one or more of the second set of one ormore optical aberrations in operation 584). In one example, the valuesfor the second set of one or more optical aberrations can be determinedusing the aberration calculator (156 in FIG. 1 ) and the second inputset. In some examples, the values determined for one or more of thesecond set of one or more optical aberrations in operation 584 can beaccumulated over time and used to determine the values for the secondoptical aberration output set (e.g., the values determined for anoptical aberration in the second set in operation 584 over time can beaveraged to determine the corresponding value for the optical aberrationin the second optical aberration output set). The operation 588 may runat a slower rate compared to the operation 584. Thus, one or moreexecutions of determining the first optical aberration output set can becompleted before a single execution of determining the second opticalaberration output set is completed (allowing for the aforementionedaccumulation of data from operation 584 to determine values for thesecond optical aberration output set).

At 590, the method 580 includes applying corrections (or adjustments) tothe charged particle beam microscope system at a second frequency tomitigate the second set of one or more optical aberrations whose valueswere determined in operation 588. In one example, the second frequencyin operation 590 is different from the first frequency in operation 586.For example, the second frequency is less than the first frequency sothat the corrections in operation 586 are applied more frequently thanthe corrections in operation 590. The second frequency can be selectedbased on the time required for each iteration of operation 588 todetermine the second optical aberration set sufficiently accurately. Inone example, the corrections can be applied by varying electricalcurrents supplied to one or more electro-optical components of thecharged particle beam microscope system.

At 591, the method 580 can include determining if the values determinedin any of operations 584 and 588 exceed predetermined thresholds. If thevalues exceed predetermined thresholds, the method can return tooperation 583 to acquire more images of the sample with the correctionsapplied to the charged particle beam microscope system.

At 592, the method 580 can include acquiring an image of the sample withthe corrected charged particle beam microscope system. In some cases,the image can be acquired when it is determined that the aberrationvalues determined in operations 584 and 588 do not exceed predeterminedthresholds. After acquiring the image, the method 580 can return tooperation 582 to process another area of the sample or the same area ofthe sample. In other examples, operation 582 can be performed inparallel to other operations in the method 580 (i.e., determiningaberration values and making corrections to the microscope system can beperformed while moving the sample relative to the microscope system).

FIG. 6 illustrates an example method 600 for acquiring an image of asample with a charged particle beam microscope system with monitoringand correction of aberrations. As noted above for FIG. 2A and the othersof the accompanying flow diagrams, operations are illustrated once eachand in a particular order in FIG. 6 , but the operations may bereordered and/or repeated as desired and appropriate (e.g., differentoperations performed may be performed in parallel, as suitable). Forexample, the method 600 may include performing any suitable combinationsof the operations presented therein in parallel.

At 610, with the sample disposed along the optical axis of the chargedparticle beam microscope system, the method 600 includes moving to anarea of interest on the sample.

At 620, the method 600 includes measuring values of aberrations in thecharged particle beam microscope system according to operations 210-230in FIG. 2A.

At 630, the method 600 includes monitoring changes in the opticalaberrations over time. The changes in the optical aberrations can bepresented in a display, for example, in the form of trend graphs. In analternative implementation of the method, monitoring of the changes inthe optical aberrations could be omitted.

At 640, the method 600 includes determining if any of the aberrationvalues exceed a predetermined threshold. If any of the aberration valuesexceed a predetermined threshold, the method 600 continues at operation650. Otherwise, the method 600 continues at operation 660.

At 650, the method 600 includes determining corrections for theaberrations exceeding predetermined thresholds and applying thecorrections to the optical column of the charged particle beammicroscope system. After applying the corrections, the method 600returns to operation 620 to repeat measurement of the aberration values.This can continue until there are no aberration values that exceedpredetermined thresholds.

At 660, the method 600 includes acquiring an image of the sample withthe corrected charged particle beam microscope system. After acquiringthe image, the method 600 can return to operation 610 to process anotherarea of the sample.

The operations in example methods 500, 580, 600 can be controlled by acomputer program, which can be stored on one or more computer-readablestorage media and executed by one or more processors of a computingsystem. The computer-readable storage media can be non-transitory (e.g.,volatile memory such as DRAM or SRAM, nonvolatile memory such asmagnetic storage, optical storage, or the like) and/or tangible.

FIG. 7 is a block diagram of a generalized example computing system (orcomputing environment) that may be used in the methods described herein.In some examples, the methods can use a single computing system. Inother examples, the methods can use multiple computing systems, whichmay communicate over a network.

With reference to FIG. 7 , the computing system 700 includes one or moreprocessing units 710, 715 and memory 720, 725. In FIG. 7 , this basicconfiguration 730 is included within a dashed line. The processing units710, 715 execute computer-executable instructions. A processing unit canbe a general-purpose central processing unit (CPU), processor in anapplication-specific integrated circuit (ASIC) or any other type ofprocessor. In a multi-processing system, multiple processing unitsexecute computer-executable instructions to increase processing power.For example, FIG. 7 shows a central processing unit 710 as well as agraphics processing unit or co-processing unit 715. The tangible memory720, 725 may be volatile memory (e.g., registers, cache, RAM),non-volatile memory (e.g., ROM, EEPROM, flash memory, etc.), or somecombination of the two, accessible by the processing unit(s). The memory720, 725 stores software 780 implementing one or more innovationsdescribed herein, in the form of computer-executable instructionssuitable for execution by the processing unit(s). Software 780 caninclude, for example, one or more of the image shift tracker 152, theaberration calculator 156, the aberration corrector 168, or the logic ofthe methods 500, 580, 600.

A computing system may have additional features. For example, thecomputing system 700 can include storage 740, one or more input devices750, one or more output devices 760, and one or more communicationconnections 770. An interconnection mechanism (not shown) such as a bus,controller, or network interconnects the components of the computingsystem 700. Typically, operating system software (not shown) provides anoperating environment for other software executing in the computingsystem 700, and coordinates activities of the components of thecomputing system 700.

The tangible storage 740 may be removable or non-removable, and includesmagnetic disks, magnetic tapes or cassettes, CD-ROMs, DVDs, or any othermedium which can be used to store information in a non-transitory wayand which can be accessed within the computing system 700. The storage740 stores instructions for the software 780 implementing one or moreinnovations described herein.

The input device(s) 750 may be a touch input device such as a keyboard,mouse, pen, or trackball, a voice input device, a scanning device, oranother device that provides input to the computing system 700. Theoutput device(s) 760 may be a display, printer, speaker, CD-writer, oranother device that provides output from the computing system 700.

The communication connection(s) 770 enable communication over acommunication medium to another computing entity. The communicationmedium conveys information such as computer-executable instructions,audio or video input or output, or other data in a modulated datasignal. A modulated data signal is a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia can use an electrical, optical, RF, or other carrier.

In view of the above described implementations of the disclosed subjectmatter, this application discloses the additional examples enumeratedbelow. It should be noted that one feature of an example in isolation ormore than one feature of the example taken in combination and,optionally, in combination with one or more features of one or morefurther examples are further examples also falling within the disclosureof this application.

Example 1 is a method including, with a charged particle beam microscopesystem operating in a transmission imaging mode, directing a chargedparticle beam to a sample including applying a time series of beam tiltsin a pattern to the charged particle beam to produce a sequence ofimages, wherein at least some of the images in the sequence of imagesare captured while the charged particle beam is transitioning betweenone beam tilt in the time series of beam tilts and a sequentiallyadjacent beam tilt in the time series of beam tilts, and wherein thepattern is configured to induce image changes between the images in thesequence of images that are indicative of optical aberrations in thecharged particle beam microscope system. The method includes capturingthe sequence of images.

Example 2 includes the subject matter of Example 1 and further specifiesthat the pattern is a periodic pattern.

Example 3 includes the subject matter of Example 2 and further specifiesthat the periodic pattern is a Lissajous figure.

Example 4 includes the subject matter of any of Examples 1-3 and furtherincludes generating a set of one or more optical aberration values forthe charged particle beam microscope system from the sequence of images.

Example 5 includes the subject matter of Example 4, wherein the imagechanges comprise induced image shifts, and wherein generating the set ofone or more optical aberration values for the charged particle beammicroscope system from the sequence of images comprises measuring theinduced image shifts between successive images in the sequence ofimages.

Example 6 includes the subject matter of Example 5, wherein generatingthe set of one or more optical aberration values for the chargedparticle beam microscope system from the sequence of the images furtherincludes obtaining an inference from a neural network model trained toidentify a set of optical aberrations from input data derived from thesequence of images.

Example 7 includes the subject matter of Example 5, wherein generatingthe set of one or more optical aberration values for the chargedparticle beam microscope system from the sequence of the images furtherincludes fitting the induced image shifts to an optical aberration modeldescribing a relationship between the induced image shifts, the beamtilts, and the set of optical aberrations.

Example 8 includes the subject matter of Example 7 and further specifiesthat the optical aberration model incorporates an unknown time delaybetween the pattern and the sequence of images.

Example 9 includes the subject matter of any of Examples 4-8, whereingenerating the set of one or more optical aberration values for thecharged particle beam microscope system from the sequence of imagescomprises estimating a value for one or more of defocus, two-foldastigmatism, three-fold astigmatism, four-fold astigmatism, five-foldastigmatism, sixfold astigmatism, axial coma, fifth order axial coma,spherical aberration, sixth order spherical aberration, star aberration,sixth order star aberration, three-lobe aberration, and rosetteaberration.

Example 10 includes the subject matter of any one of Examples 4-9 andfurther includes determining optical corrections for the chargedparticle beam microscope system based on the set of one or more opticalaberration values.

Example 11 includes the subject matter of Example 10 and furtherincludes applying the optical corrections for the charged particle beammicroscope system to one or more optical components of the chargedparticle beam microscope system.

Example 12 includes the subject matter of Example 10 and furtherincludes adjusting the pattern based on one or more of accuracy of theset of one or more optical aberration values, quality of the images inthe sequence of images, and external disturbances while capturing thesequence of images.

Example 13 includes the subject matter of any one of Examples 4-12 andfurther includes monitoring changes in the set of one or more opticalaberrations over time.

Example 14 is a method including receiving, by one or more processors, asequence of images captured while applying a time series of beam tiltsin a periodic pattern to a charged particle beam transmitted along anoptical axis of an optical column of a charged particle beam microscopesystem and through a sample disposed along the optical axis. The methodincludes estimating, by the one or more processors, values for one ormore optical aberrations produced by the optical column from thesequence of images.

Example 15 includes the subject matter of Example 14 and furtherincludes determining one or more optical corrections for one or moreoptical components in the optical column; and selectively applying theone or more optical corrections to the one or more optical components.

Example 16 includes the subject matter of any one of Examples 14-15 andfurther specifies that the periodic pattern is a Lissajous pattern.

Example 17 is a system including a charged particle source that emits acharged particle beam; an optical column defining an optical axis andcomprising one or more optical components configured to form the chargedparticle beam into a field of view of a sample disposed along theoptical axis; a pattern generator configured to generate a pattern; oneor more beam deflectors disposed along the optical axis and controllableby the pattern to apply a time series of beam tilts to the chargedparticle beam; an imaging sensor positioned along the optical axis tocapture images of the sample; one or more processors; and one or morecomputer-readable storage media storing instructions that when executedby the one or more processors cause the one or more processors toreceive a sequence of images captured by the imaging sensor duringapplication of the time series of beam tilts and cause the one or moreprocessors to generate a set of one or more optical aberration valuesfor the optical system from the sequence of images, wherein at leastsome of the images in the sequence of images are captured duringmovement of the charged particle beam between consecutive tilts in thetime series of beam tilts.

Example 18 includes the subject matter of Example 17 and furtherspecifies that the one or more computer-readable storage media furtherstore instructions that when executed by the one or more computingdevices cause the one or more computing devices to determine inducedimage shifts in the sequence of images and determine the values for theone or more optical aberrations from the induced image shifts and anapplied time series of beam tilts corresponding to the induced imageshifts.

Example 19 includes the subject matter of Example 17 and furtherspecifies that the one or more computer-readable storage media furtherstore instructions that when executed by the one or more computingdevices cause the one or more computing devices to determine one or moreoptical corrections for the one or more optical components based on theset of one or more optical aberration values.

Example 20 includes the subject matter of any one of Examples 17-19, andfurther specifies that the one or more computer-readable storage mediafurther store instructions that when executed by the one or morecomputing devices cause the one or more computing devices to determineadjustments to the pattern generated by the pattern generator based on aconfidence level of the values determined for the one or more opticalaberrations.

Example 21 includes the subject matter of any one of Examples 17-20, andfurther specifies that the pattern generator is configured to generate aperiodic pattern.

Example 22 is a method of operating a charged particle beam microscopesystem including determining a first optical aberration output set,wherein the first aberration output set includes values for a first setof one or more optical aberrations in the charged particle beammicroscope system, based on a first input set that includes one or moreacquired images; applying adjustments, based on the first opticalaberration output set, to the charged particle beam microscope system ata first frequency to mitigate optical aberrations in the first set;determining values for a second optical aberration output set thatincludes values for a second set of one or more optical aberrations inthe charged particle beam system, wherein the one or more opticalaberrations of the second set are different than the one or more opticalaberrations in the first set, based on a second input set that includesat least some of the first input set or the first optical aberrationoutput; and applying adjustments, based on the second optical aberrationoutput set, to the charged particle beam microscope system at a secondfrequency to mitigate aberrations in the second set, wherein the secondfrequency is less than the first frequency.

Example 23 includes the subject matter of Example 22 and furtherspecifies that the first set of one or more optical aberrations includesdefocus.

Example 24 includes the subject matter of any one of Examples 22-23 andfurther specifies that the second set of one or more optical aberrationsincludes coma or astigmatism.

Example 25 includes the subject matter of any one of Examples 22-24,wherein determining the first optical aberration output set based on thefirst input set includes determining at least some values for one ormore optical aberrations in the second set, and wherein the at leastsome of the first optical aberration output set included in the secondinput set includes the at least some values.

1. A system, comprising: a charged particle source that emits a chargedparticle beam; an optical system defining an optical axis and comprisingone or more optical components configured to form the charged particlebeam into a field of view of a sample disposed along the optical axis; apattern generator configured to generate a pattern; one or more beamdeflectors disposed along the optical axis and controllable by thepattern to apply a time series of beam tilts to the charged particlebeam; an imaging sensor positioned along the optical axis to captureimages of the sample; one or more processors; and one or morecomputer-readable storage media storing instructions that when executedby the one or more processors cause the one or more processors toreceive a sequence of images captured by the imaging sensor duringapplication of the time series of beam tilts and cause the one or moreprocessors to generate a set of one or more optical aberration valuesfor the optical system from the sequence of images, wherein at leastsome of the images in the sequence of images are captured duringmovement of the charged particle beam between consecutive beam tilts inthe time series of beam tilts.
 2. The system of claim 1, wherein the oneor more computer-readable storage media further store instructions thatwhen executed by the one or more processors cause the one or moreprocessors to determine induced image shifts in the sequence of imagesand determine the set of one or more optical aberration values from theinduced image shifts and an applied time series of beam tiltscorresponding to the induced image shifts.
 3. The system of claim 1,wherein the one or more computer-readable storage media further storeinstructions that when executed by the one or more processors cause theone or more processors to determine one or more optical corrections forthe one or more optical components based on the set of one or moreoptical aberration values.
 4. The system of claim 1, wherein the one ormore computer-readable storage media further store instructions thatwhen executed by the one or more processors cause the one or moreprocessors to determine adjustments to the pattern generated by thepattern generator based on one or more of an accuracy of the set of oneor more optical aberration values, quality of the images in the sequenceof images, and external disturbances while capturing the sequence ofimages.
 5. The system of claim 1, wherein the pattern generator isconfigured to generate a periodic pattern.
 6. A method, comprising: witha charged particle beam microscope system operating in a transmissionimaging mode, directing a charged particle beam to a sample includingapplying a time series of beam tilts in a pattern to the chargedparticle beam to produce a sequence of images, wherein at least some ofthe images in the sequence of images are captured while the chargedparticle beam is transitioning between one beam tilt in the time seriesof beam tilts and a sequentially adjacent beam tilt in the time seriesof beam tilts, and wherein the pattern is configured to induce imagechanges between the images in the sequence of images that are indicativeof optical aberrations in the charged particle beam microscope system;and capturing the sequence of images.
 7. The method of claim 6, whereinthe pattern is a periodic pattern.
 8. The method of claim 7, wherein theperiodic pattern is a Lissajous figure.
 9. The method of claim 6,further comprising generating a set of one or more optical aberrationvalues for the charged particle beam microscope system from the sequenceof images.
 10. The method of claim 9, wherein the image changes compriseinduced image shifts, and wherein generating the set of one or moreoptical aberration values for the charged particle beam microscopesystem from the sequence of images comprises measuring the induced imageshifts between successive images in the sequence of images.
 11. Themethod of claim 10, wherein generating the set of one or more opticalaberration values for the charged particle beam microscope system fromthe sequence of images further comprises fitting the induced imageshifts to an optical aberration model describing a relationship betweenthe induced image shifts, the beam tilts, and the set of one or moreoptical aberrations.
 12. The method of claim 10, wherein the opticalaberration model incorporates an unknown time delay between the patternand the sequence of images.
 13. The method of claim 9, whereingenerating the set of one or more optical aberration values for thecharged particle beam microscope system from the sequence of imagescomprises estimating a value for one or more of defocus, two-foldastigmatism, three-fold astigmatism, four-fold astigmatism, five-foldastigmatism, sixfold astigmatism, axial coma, fifth order axial coma,spherical aberration, sixth order spherical aberration, star aberration,sixth order star aberration, three-lobe aberration, and rosetteaberration.
 14. The method of claim 9, further comprising determiningoptical corrections for the charged particle beam microscope systembased on the set of one or more optical aberration values.
 15. Themethod of claim 14, further comprising applying the optical correctionsfor the charged particle beam microscope system to one or more opticalcomponents of the charged particle beam microscope system.
 16. Themethod of claim 9, further comprising adjusting the pattern based on oneor more of accuracy of the set of one or more optical aberration values,quality of the images in the sequence of images, and externaldisturbances while capturing the sequence of images.
 17. A method ofoperating a charged particle beam microscope system, the methodcomprising: determining a first optical aberration output set, whereinthe first aberration output set includes values for a first set of oneor more optical aberrations in the charged particle beam microscopesystem, based on a first input set that includes one or more acquiredimages; applying adjustments, based on the first optical aberrationoutput set, to the charged particle beam microscope system at a firstfrequency to mitigate optical aberrations in the first set; determiningvalues for a second optical aberration output set that includes valuesfor a second set of one or more optical aberrations in the chargedparticle beam system, wherein the one or more optical aberrations of thesecond set are different than the one or more optical aberrations in thefirst set, based on a second input set that includes at least some ofthe first input set or the first optical aberration output; and applyingadjustments, based on the second optical aberration output set, to thecharged particle beam microscope system at a second frequency tomitigate aberrations in the second set, wherein the second frequency isless than the first frequency.
 18. The method of claim 17, wherein thefirst set includes defocus.
 19. The method of claim 17, wherein thesecond set includes coma or astigmatism.
 20. The method of claim 17,wherein determining the first optical aberration output set based on thefirst input set includes determining at least some values for one ormore optical aberrations in the second set, and wherein the at leastsome of the first optical aberration output set included in the secondinput set includes the at least some values.